1. Field of the Invention
This invention relates to a transfer unit for a full color copying machine.
2. Description of the Prior Art
Generally, in a full color copying machine, three separate images are formed on a photoconductor belt by the use of toner of three primary colors, yellow, magenta, and cyan. The toner images are transferred onto copy paper by means of a transfer unit. The transfer unit has a transfer belt the back surface of which is supported by a metal backup roller. The three primary colored toner images formed on the photoconductor belt are transferred onto the transfer belt in such a way that the images are superimposed with one another. A metal transfer roller to which a predetermined transfer voltage is applied is disposed opposite to the backup roller across the transfer belt. Copy paper is transported between the transfer roller and the transfer belt onto which the toner image has been transferred, the transfer roller pressing the copy paper against the backup roller with the transfer belt placed therebetween, while an electric field necessary for the toner image on the transfer belt to be transferred is created by the voltage applied to the transfer roller, thus transferring the toner image from the transfer belt onto the copy paper.
In the above mentioned transfer unit, since the backup roller is made of metal, the voltage applied to the transfer roller causes current to flow between the two rollers, thus creating an electric field. Since an electric field necessary for transferring a toner image is created between the two rollers, a relatively high voltage is required to form an electric field necessary for transferring a toner image, resulting in a larger total current flowing between the transfer roller and the backup roller. As a result, when no copy paper exits between the two rollers, or depending on the kind of copy paper when copy paper does exits between the two rollers, there arises a possibility of the transfer belt suffering a dielectric breakdown. Also, depending on the change in the environment surrounding the transfer unit, e.g., change of temperature, humidity, etc., the total current required to form an electric field necessary for transferring a toner image may substantially wander, resulting in the inconsistency of the quality of the toner image transferred to the copy paper.